Method and apparatus for determining color, in particular of a dental prosthesis

ABSTRACT

The invention can be used to determine the color to be given to a dental prosthesis, for example, on the basis of color measurements performed on adjacent teeth in the mouth of the patient. An optical fiber instrument picks up light reflected from a tooth and transmits it to the inlet of a spectrocolorimeter which associated with a microprocessor in order to determine the diffuse spectral reflectance of the tooth and to calculate the tristimulus values of its apparent color under various different types of illumination. The invention is particularly suitable for determining the color of dental prostheses.

The invention relates to a method and to apparatus for determining thecolor to be given to an object, in particular a ental prosthesis, on thebasis of the color of another object, in particular the adjacent teethin the mouth of the patient.

BACKGROUND OF THE INVENTION

It is essential for a dental prosthesis (e.g. a crown, a bridge, animplant, etc.,) to be as close as possible in color to the adjacentteeth.

At present, the color of a dental prosthesis is determined at sight, ingeneral by a dentist comparing color samples with the teeth of thepatient. This procedure gives results that necessarily depend on thedentist's capacity to distinguish between colors which are very similar,which is sufficient reason for results that are not always verysatisfactory.

It may also happen that a prosthesis color selected in this way is veryclose to the color of the adjacent teeth so long as the teeth areilluminated by a light source giving a generally uniform spectrum, butthat the prosthesis color appears quite different when the teeth areilluminated by a light source such as a fluorescent tube which radiatesa spectrum concentrated in a few lines (a phenomenon sometimes known asmetamerism).

Colorimeters exist which measure the color of an object by using filtersto determine the tristimulus values X, Y, and Z of the object's color,thereby enabling said color to be reproduced by mixing three primarycolors. However, a knowledge of the tristimulus values applicable forany given illumination is not sufficient for specifying the values underdifferent illumination. Thus, in order to obtain good results usingcolorimeters it is necessary to take a large number of color measurmentsunder different lighting conditions, which is a considerable problem.

The invention seeks to mitigate this problem.

An aim of the invention is to provide a method and apparatus fordetermining the apparent colors of an object corresponding to variousdifferent types of illumination, on the basic of a single measurementperformed under unspecialized illumination.

Another aim of the invention is to provide a method and apparatus of theabove-mentioned type suitable for performing such measurements onobjects which are difficult of access, such as a person's teeth.

SUMMARY OF THE INVENTION

The present invention provides a method of determining the color, inparticular of a dental prosthesis, with reference to the color of anidentical object such as adjacent teeth in the mouth of a patient, themethod consisting in: picking up color information in situ by means ofone or more optical fibers suitable for being inserted into the mouth ofthe patient to pick up light reflected from a tooth adjacent to theemplacement for the prosthesis; in transmitting the reflected light byoptical fiber to a spectrocolorimeter; and in determining the diffusespectral reflectance D(λ) of the tooth, and then in calculating thevarious apparent colorations of the tooth corresponding to differenttypes of light by which it may be illuminated, in order to define theprosthesis color having the best possible match from the esthetic pointof view with the adjacent teeth.

After being suitably sampled, the diffuse spectral reflectance of theilluminated tooth can be used to calculate the tristimulus values of thetooth's coloration under any possible type of illumination; and inpractice this is done for standardized types of light sourcerepresentative of various possible types of illumination (e.g. an equalenergy source, C.I.E. standard illuminants, a spectrum line source,etc.).

The invention thus makes it possible to obtain the tristimulus values ofthe apparent colors of a tooth under different types of illuminationfrom a single measurement, and thereby makes it possible to select aprosthesis color with tristimulus values that are as close as possibleto those of the apparent colors of the tooth.

Diffuse spectral reflectance measurements may be performed, and theapparent colors may be determined for several of the patient's teeth andfor several prosthesis color test samples, thereby enabling the samplewhose apparent colors are closet to those of the patient's teeth to bedetermined.

Advantageously, when determining which prosthesis color to use, moreweight is given to appearance under daylight than under other types ofillumination.

The invention can also be used to display an image on a graphics screenshowing the colors of a set of adjacent teeth (including both naturalteeth and prosthesis teeth) as they would appear under various differenttypes of illumination.

It is thus possible to verify whether the selected prosthesis color is agood match to the color of the patient's other teeth under differentlighting conditions.

The invention also provides apparatus for determining the color to begiven, in particular to a dental prosthesis with reference to the colorof at least one adjacent tooth in the mouth of a patient, said apparatuscomprising: an instrument for picking up color information in situ, saidinstrument being suitable for insertion into the mouth of the patientand including optical fiber means for picking up and for transmittinglight reflected by a tooth; a spectrocolorimeter having an optical fiberinput path connected to said pick-up and transmission means, saidspectrocolorimeter including means measuring and determining the diffusespectral reflectance of the tooth; and calculating means associated withthe spectrocolorimeter for determining the various apparent colorationsof the tooth corresponding to different types of illumination.

The instrument for picking up color information includes an opticalfiber associated with a light source for illuminating the tooth,together with an optical fiber for picking up and for transmitting thelight reflected by the tooth, with optical focusing means enabling localmeasurements to be performed on a portion of a tooth, and/or with meansfor homogenizing the incident or the reflected light, e.g. anintegrating microsphere.

Advantageously, the spectrocolorimeter is of the type comprising aspectrometer having a mosaic of photodetector elements associated with amicroprocessor and mounted on an optoelectronic card which also mountsat least two optical fiber measurement paths leading to the inlet of thespectrometer and provided with shutters for switching the measurementpaths, with means for calibrating the photodetector wavelengths, andelectronic circuits for reading the photodetectors.

One of the spectrometer measurement paths may then be associated withthe light source, while the other measurement path is connected to theinstrument for picking up color information in order to receive lightreflected by the illuminated tooth.

The microsprocessor may be associated with a graphics terminal in orderto display sets of adjacent teeth (including both natural teeth andprosthesis teeth), and to show their colors as they would appear underdifferent types of illumination.

BRIEF DESCRIPTION OF THE DRAWING

In the following description given by way of example, reference is madeto the accompanying drawing, in which:

FIG. 1 is a diagrammatic view of an apparatus in accordance with theinvention; and

FIG. 2 is a diagrammatic view on a larger scale of a color informationpick-up instrument shown facing a tooth.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the embodiment shown in FIG. 1, the apparatus in accordance with theinvention is essentially constituted by associating a spectrocolorimeterof the type described in French patent application No. 85 15351 filedOct. 16, 1985, with an instrument for picking up light in situ, togetherwith an unspecialized light source.

The spectrocolorimeter is essentially constituted by associating acontrol microprocessor 10 and an opto-electronic card 12 whose outlineis represented in dashed lines and on which there are mounted: adiffraction grating type spectrometer 14, for example using Littrow typeoptics including an inlet slot 16 and a strip 18 of photodetectors 20which may, for example, be silicon photodiodes.

These photodetectors 20 are associated with electronic circuits given ageneral reference 22 which are mounted on the card 12 and which include:circuits, e.g. multiplex type circuits, for reading detector charge; ananalog-to-digital converter; a programmable clock for determining theintegration time of the photodetectors 20; buffer memories; and circuitsfor processing the digitized data.

The card 12 also carries means for calibrating the photodetectors 20 inwavelength, which means are constituted in this case by twolight-emitting diodes (LEDs) 24, which are DC powered under the controlof the microprocessor 10 and which are connected to the inlet slot 16 ofthe spectrometer via optical fibers 26 constituting two calibrationpaths. Each LED 24 emits light radiation of a given wavelength whenpowered, and the radiation wavelengths of the two LEDs are suitably farapart from each other in the visible spectrum.

Two optical fiber measurement paths 28 are also provided on the card 12,with one end of each measurement path being disposed at the inlet slot16 to the spectrometer, and with the other end of each measurement pathbeing connected via a connector 30 mounted on the card 12 to ameasurement path 32a, 32b formed by an optical fiber which may be verylong, if so desired.

Each measurement path 28 mounted on the card 12 is also fitted with ashutter 34 which is DC powered under the control of the microprocessor10 and which serves to open or close the corresponding measurement path28.

A light source 36 which may be constituted, for example, by a lampemitting reference spectrum radiation, is connected via an optical fiber38 and a Y-coupler 40 firstly to the input of the measurement path 32aof the spectrometer 12, and secondly via an optical fiber 42 to a lightpick-up instrument 44 which is also connected to the other spectrometermeasurement path 32b.

The instrument 44 contains two optical fibers 46 and 48 with arerespectively connected to the optical fibers 42 and 32b or which areconstituted by extensions thereof, and which open out freely at the endof the instrument 44. In practice, the instrument 44 is in the form of adental instrument with a diameter of a few millimeters so that it can beeasily inserted into the mouth of a patient and brought up to a tooth50.

The instrument 44 may optionally include light focusing means (notshown) enabling local information to be obtained from a point on atooth. The instrument may also include means for homogenizing theincident or the reflected light, e.g. an integrating microsphereassociated with the fiber 46 or 48, thereby overcoming variations due tovarying incidence. Optical information may be picked up using a 0/45 ora 45/0 geometry (incident light normal, reflected light at 45°, or viceversa) or using a O/d or a d/O geometry (incident light normal andreflected light taken up by the microsphere, or vice versa).

Further, as already described in French patent application No. 85 15351,the ends of the optical fibers forming the measurement paths 28 and thecalibration paths 26 are superposed on one another in the inlet slot 16to the spectrometer. The detectors 20 must therefore be of adequateheight, i.e. their height must correspond to the height of thesuperposed ends of the optical fibers. When the spectrometer 14 usesLittrow optics having a magnification ratio of 1/1, the height of thedetectors 20 must be not less than the height of the superposed ends ofthe four optical fibers constituting the measurement and calibrationpaths, in other words the height must be equal to four times thediameter of a single optical fiber.

Further, a graduated density compensation filter 52, referred to as afocal plane corrector, is placed over the photodetectors 20 so that theyall receive energy flux of the same order of magnitude, and preferablyso that they all receive substantially equal energy flux.

Optical fibers transmit poorly in the blue region of the spectrum, sothat the energy flux of spectrum components entering the spectrometer inthe blue is much less than the energy flux of the red components, andattenuation also increases with increasing length of the optical fibersconstituting the measurement paths. The focal plane corrector 52 servesto reestablish substantially uniform energy flux over the photodetectors20. Further, its use is preferable to the use of a set of blue filterswhich pass only a fraction of the incident energy flux.

The above-described apparatus operates as follows:

The operations of calibrating the photodetectors 20 may be performedautomatically in a periodic manner under the control of themicroprocessor 10, or else on user request. To do this, the shutters 34on the measurement paths 28 are closed and measurements are performed byturning on one of the LEDs 24 while the other is off, and then byturning on the other LED 24 while the first is off. These measurementsare repeated regularly at predetermined time intervals of given length,with the results of the first measurements being stored in memory andcompared with the results of subsequent measurements, in order to verifythat they match and to automatically recalibrate the strip ofphotodetectors 20, if necessary.

In order to perform a spectrum measurement, both LEDs 24 are held off,and one of the measurement paths 28 is opened while the othermeasurement path 28 is closed. The energy flux received by thephotodetectors 20 is measured and compared with the saturationthreshold, in order to determine the time during which the photodetectorshould integrate the received signal so as to obtain as large a singalas possible below the saturation threshold. In general, the integrationtime varies between about one second and one millisecond, and it isdetermined by a programmable clock, thereby making it possible toautomatically match the integration time to the value of the receivedenergy flux so as to obtain a signal representing 90% of the dynamicrange of the photodetectors.

The signals from the detectors 20, i.e. the charges thereon, are read bymultiplex type read circuits, operating at a constant read frequencyequal to the maximum red frequency for the detectors of the strip, i.e.the frequency which corresponds to the minimum value of the integrationtime. When the photodetectors are read, the integration is reset to zerofor the next measurement. The signals read from the detectors aredigitized by an analog-to-digital converter, and are then processed.Photodetector noise is measured by closing all of the measurement paths.The measured noise is automatically substracted from the measurementsignal obtained by opening a measurement or a reference path.

The order to determine the apparent colorations of a tooth, thefollowing procedure is followed:

The light pick-up instrument 44 is placed in the patient's mouthopposite the zone of tooth whose color is to be determined. The tooth isilluminated by the light source 36 via the optical fiber 42, 46 whichends at the end of the instrument 44 facing the tooth 50. The lightreflected by the tooth is modified by the color of the tooth and ispicked-up by the optical fiber 48 and is then transmitted by themeasurement path 32b to the inlet slot 16 of the spectrometer 14.

Initially, the measurement path 32a associated with the lamp 36 is openin order to perform a spectrometric measurement of the radiation emittedby the lamp, while the measurement path 32b associated with the tooth 50is closed. Then the measurement path 32a is closed while the measurementpath 32b is opened in order to perform a spectrometric measurement ofthe light reflected by the tooth 50. The ratio of these two measurementsis used to determine the diffuse spectral reflectance D(λ) of the tooth.

The tristimulus values X, Y, and Z of the color of the tooth aredetermined using the following equations: ##EQU1## where: λ iswavelength,

D(λ) is the diffuse reflectance,

S(λ) is a spectral coefficient depending on the illumination source, and

x(λ), y(λ), and z(λ) are the spectral tristimulus values for the C. I.E. colorimetric reference observer.

The chromaticity co-ordinates x, y, and z of the color of the tooth aregiven by the following equations: ##EQU2##

While the diffuse spectral reflectance D(λ) is available, wavelengthsampling is performed step-by-step over a range of 5 nm to 10 nm, forexample, and the tristimulus values X, Y, and Z are calculated using theabove equations for a corresponding value of S(λ). The S(λ) curves forstandardized reference light sources are known at the same sampleintervals as are used for sampling D(λ), as are the sampled values ofx(λ), y(λ), and z(λ).

It is thus possible to determine the tristimulus values of the apparentcolorations of a tooth under various different lighting conditions froma single measurement of its diffuse spectral reflectance.

The same operations can be repeated by placing the end of the pick-upinstrument 44 opposite prosthesis color test samples, of the kind keptby dentists and prosthesis makers. By comparing the results obtained,i.e. the apparent colorations under different types of lighting, it ispossible to determine which one of the test samples is closest in colorto the tooth under different types of illumination. In practice, whenperforming such comparisons, most weight is given to the colors thatappear under natural light.

These operations of determining the diffuse spectral reflectance and ofcalculating the tristimulus values can naturally be performed for otherzones of a patient's tooth or for other teeth adjacent to theprosthesis.

The results of these measurements and of these calculations can bedisplayed on a graphics terminal 54 associated with a microprocessor 10,with a set of teeth being displayed on the terminal having the sameshapes as the patient's teeth and as the prosthesis teeth, and havingcolors that correspond to the apparent colors as determined for variousdifferent types of lighting. This display makes it possible to checkvery quickly whether a pleasing color match has been obtained betweenthe test samples and the patient's own teeth.

Apparatus in accordance with the invention can thus be used in a rapidand reliable manner to determine which color should be used for a dentalprosthesis, as a function of the color of the teeth which are adjacentto the prosthesis in the mouth of the patient.

Dental prostheses are often made of resin which is colored by pigmentswhich are embedded therein quantities that vary as a function of thecolor to be obtained. The invention can be used not only to determinethe color to be given to the resin to a high degree of accuracy, butalso to determine the corresponding quantities of pigment, and thus toautomate the resin-preparing process.

The invention is also applicable to other technical fields in which thecolor of one article must be matched to the color of another.

We claim:
 1. A method for determining a color for a dental prosthesis byreferencing the color of an adjacent tooth in the mouth of a patient,comprising the steps of:inserting instrument means hving at least oneoptical fiber in the mouth of the patient; illuminating through theinstrument means at least one tooth adjacent to an area of emplacementfor a dental prosthesis; picking up through the instrument means colorinformation formed by the illuminating light reflecting off the adjacenttooth; transmitting through the optical fiber the reflected light to aspectrocolorimeter; determining a diffuse spectral reflectance of theadjacent tooth; calculating various colorations for the dental prothesisfrom the diffuse spectral reflectance, the various colorationscorresponding to different types of light by which the dental prothesismay be illuminated; and displaying a set of teeth representing theadjacent tooth and the dental prothesis with respective colorationsunder the different types of light.
 2. The method according to claim 1,wherein said steps of calculating and displaying of the apparentcolorations for the dental prothesis and the adjacent tooth are repeatedfor a plurality of color test samples to determine which sample hasapparent colorations closest to the apparent colors of the adjacenttooth.
 3. The method according to claim 1, further comprising the stepsof:performing a spectrometric measurement with light emitted from alight source; performing a spectrometric measurement with the reflectedlight from the adjacent tooth; and determining the diffuse spectralreflectance of the adjacent tooth using the ratio of the twospectrometric measurements.
 4. A method according to claim 1, whereinmost weight is given to the apparent color of the prosthesis undernatural light when defining the color of the prosthesis.
 5. A methodaccording to claim 1, wherein the set of teeth representing theprosthesis and the adjacent tooth of the patient are displayed on agraphics terminal to show their respective apparent colors.
 6. Anapparatus for determining a color for a dental prosthesis by referencingthe color of at least one adjacent tooth in the mouth of a patient, saidapparatus comprising:instrument means, suitable for insertion into themouth of a patient, for picking up color information of a tooth adjacentan area of emplacement of a dental prothesis; optical fiber meansassociated with said instrument means for illuminating the adjacenttooth and for picking up reflected light from the adjacent tooth; aspectrocolorimeter having an optical fiber path connected to saidoptical fiber means for receiving the reflected light from said opticalfiber means, said spectrocolorimeter including means for measuring anddetermining diffuse spectral reflectance of the adjacent tooth;calculating means associated with said spectrocolorimeter fordetermining, under different types of illuminations, various apparentcolorations of the adjacent tooth; and a graphics display terminal fordisplaying the dental prothesis and the adjacent tooth as a set of teethwith respective apparent colorations, wherein the color of the dentalprothesis is chosen from a plurality of displayed color test samples. 7.Apparatus according to claim 6, wherein said instrument means forpicking up color information includes an optical fiber associated with alight source for illuminating the tooth, together with an optical fiberfor picking up and for transmitting the light reflected by the tooth. 8.Apparatus according to claim 6, wherein the spectrocolorimeter is of thetype comprising a spectrometer having an inlet and a mosaic ofphotodetector elements associated with a microprocessor and mounted onan optoelectronic card which also mounts at least two optical fibermeasurement paths leading to the inlet of the spectrometer and providedwith shutters for switching the measurement paths, means for wavelengthcalibrating the photodetector element, and electronic circuits forreading the photodetector element.
 9. Apparatus according to claim 8,wherein one of the spectrometer measurement paths is associated with alight source and wherein the other measurement path is connected to theinstrument means for picking up color information for receiving lightreflected by the illuminated tooth.